The present invention relates to signal detection circuitry and more particularly to circuitry for distinguishing repetitive waveforms having differing amplitudes.
The invention will be described in the environment of a liquid crystal display (LCD) driver though there are many and varied applications. One such other application is the demodulation of digital amplitude modulated (DAM) carriers, i.e., carriers which are modulated by two discrete amplitudes representing binary ones and zeros. Apparatus of this type is described in U.S. Pat. No. 3,290,520, by L. P. Wennik, entitled "Circuit for Detecting Amplitude Threshold with Means to Keep Threshold Constant". The Wennik circuit is a differential amplifier biased to be substantially unresponsive to peak-to-peak input signals whose amplitude is within a preset threshold range, and to produce an output signal when the input signal exceeds this range. This circuit has a shortcoming in that it is sensitive to noise riding on the input signal and may produce an output signal responsive to a noisy signal whose amplitude is generally within the preset range. It is an object of the present invention to produce a substantially noise insensitive signal amplitude detection circuit.
In LCD driver circuitry, the detection circuit serves to determine when the circuitry is being operated with an internal oscillator or an externally generated clocking signal. Nominally, the internal oscillator produces an oscillatory signal of lesser amplitude than the external signal. The internal oscillator operates at a frequency far in excess of the external signal and is divided down to the external clock rate. The detection signal serves to multiplex the output of the counter or the external clock signal to the LCD output driver.